1. Field of the Invention
The subject invention relates to electrically conductive composite materials and, more particularly, to graphite epoxy composite materials formed into conductive structures.
It has long been recognized that graphite is at least a semiconductor of DC and RF energy. However, the only heretofore practical application of this characteristic has been the use of chopped fibers in an epoxy matrix for RFI shielding and as an RF reflector surface, i.e., parabolic reflectors.
2. Description of Related Art
A main design consideration for almost every structure, and especially airborne and spaceborne structures, is weight. The designer needs materials having a certain strength, while at the same time having as little weight as possible. Increasingly, designers seek to combine multiple functions in a single structure. For example, a structure which provides a necessary antenna configuration, while being at the same time electrically conductive, provides required mechanical and electrical functions in a single structure. A further requirement is that such structure be as lightweight as possible.
As an example, in the case of a dipole antenna to be carried aboard a spacecraft, the antenna must be as lightweight as possible, in one case not exceeding 0.5 lb. Such structures must, for example, be able to withstand harsh mechanical vibrations associated with satellite launch environments. Such requirements imply a high stiffness-to-weight ratio. While aluminum and steel can probably meet strength and electrical conductivity requirements, they, in many cases, are far too heavy to meet mission weight limitations.
As another example, aircraft wings may, in addition to providing the necessary airfoil for lift purposes, house radio or radar antennas, and wing heaters for deicing purposes. While typically these three functions are provided by different materials which are interconnected in some manner, the mere use of three different materials results in a certain weight accumulation. It would be an advance in the art if all three functions could be provided by a single material, and if such material were more lightweight than prior techniques. Once again, a high stiffness-to-weight ratio would be required to meet the stresses placed on an aircraft wing.
Some investigation into the use of conducting plastics has been performed. However, such presently-known materials suffer from severe disadvantages, such as poor strength when highly conductive, or poor conductivity when strength is increased. Many are not stable under extreme temperature ranges, some degrade relatively rapidly in the presence of water, and most, if not all, do not possess sufficient electrical conductivity for many applications.
It would be an advance in the art to provide a material having a high stiffness-to-weight ratio, having relatively high electrical conductivity, and having relatively low weight.